Electric motor reversing control



Nov. 16, 1948. RElLLY 2,454,206

ELECTRIC MOTOR REVERSING CONTROL Filed June 16, 1945 zzz 76 7 mo 22 I; /02

INVENTOR a? 4 i JbcKEEZ/l/y.

BY ATTORNEY WITNESSES:

Patented Nov. 16, 1 948 UNITED STATES ATENT orrics ELECTRIC MOTOR REVERSING CONTROL Application June 16, 1945', SerialNo. 599,838

6 Claims.

Thisinvention relates to electrical systems and in particular to arc furnace regulating systems.

An object of this invention is to provide in an electronic arc furnace regulating system, for automatically braking the electrode positioning motor after an electrode positioning operation.

Another object of this invention is to provide an electronic control system for a reversible motor in which the motor is automatically braked after an operation without the use of moving elements.

Other objects of this invention Will become apparent from the following description when taken in conjunction with the accompanying drawing the single figure of which is a diagram matic View of the apparatus and circuits of an arc furnace regulating system embodying the teachings of this invention.

Referring to the drawing, this invention is illustrated by reference to an arc furnace IIIhaving electrodes I2, I l and I6 disposed therein to be positioned with respect to a metal charge I8. The electrodes I2, I4 and I5 are connected by conductors 20, 22 and24, respectively, to a source of supply (not shown). Since the control system and apparatus utilized in positioning each of the electrodes is the same, only the system and. apparatus associated with electrode Illis illustrated, it being understood that similar control systems are to be provided for the other electrodes.

As illustrated, a reversible motor 26 is utilized for positioning the electrode I2, the electrode I2 being connected to be raised or lowered by operation of the motor. For this purpose a winding drum 28 is disposed to be driven by the motor II), a flexible cable 38 being connected to the electrode I2 and disposed to be Wound upon the drum 28. The motor comprises an armature winding 32 and a field winding 34, the field Winding being disposed to be separately excited from a source of supply (not shown).

In order to control the direction and speed of operation of the motor 26, two sets of electric valves such as the thyratron tubes 3638- and 4042 are utilized. The valves 36, 38, 40 and 42 are provided with anodes 44, 46, 48 and 56-, respectively, connected to the terminals of the secondary windings of transformers 52 and 54 as illustrated, the primary windings of which are connected across supply conductors 56 and 58-. The valves 36, 38, 4D and 42 are also provided with cathodes 50, 62, 64' and 66, respectively, the cathodes 6i) and IE2 being connected through conductors 68-40 and I2; respectively, to conductor I4, through resistorli, conductors 78 and B0; armature winding 32, conductors 82 and 8 4, resistor 66 and the conductor 38 to the center tap 9 3- or the secondary winding of transformer 52'. In a similar manner the'cathodes 64' and are connected by conductors 92* and lit- 96, respectively, to conductor 88' through resistor 169, conductors 84 and 82; armature winding 32, conductors 8i! and I8; resistor I62 and conductor I64 to the center tap I06 of the secondary winding of transformer 54. I

The valves 36, 38; land 42 are also provided with grids I68, H6, I12 and H4, respectively. The grids lilt'and. III] are supplied with an alternating current potential from the supply conductors 56' and 58 through a transformer H6, a phase shifting circuit II 3 and a grid transformer I26. The secondary winding of the grid transformer comprises two sections I22 and I24, the

winding I22 having one end connected through.

a resistor I26, to, the grid H6 and the other end connectedthrough a conductor I28, a part of resistor I3I1and the, conductor IZto the cathode 62 of valve 38 whereas the winding 224 has one end, connected through resistor I32 to the grid I68 the other end, connected through conductor I34, resistor I36: and conductors is, iii, and 68 to thecathode 66, of the valvetfi.

Likewise,- an alternating current potential is supplied: to. the grids H2 and II4 of valves 48 and 42, respectively; from supply conductors 56 and 58through transformer I40, thexphaseshift ing circuit I42 and the grid transformer- 4,4 whichis provided with two'seconclary windings I66 and I48; The winding I46, has one end connected through resistor lfiiitolthe grid I54 of valve 42;

the other end of winding IE6 being connected 363B" and 40-42; respectively, for controlling.

the grid cathode potential of the valves in a predetermined manner. Thus one end of the re,-

sistor IBEl'is connectedthr'oug'h conductors 68 and.

Id-32' to cathodes 6i! and 62, respectively, and a flexible lead I64 connected to an intermediate tap of resistor I is connected through resistor up a tilt, conductor H38, resistor llll and conductor i3 1 to the secondary Winding E25, the conductor I58 also being connected through a part of resistor I35 and conductor R28 to the secondary winding lfill of the grid transformer 528, In the same maner one end of resistor N52 is connected through conductors ti l and ,%32 to the cathodes 6B and G l, respectively, of valves 42 and it respectively, and a flexible lead ll2 connected to an intermediate tap of resistor E62 is connected through resistor ll l, conductor H6, resistor ill; and conductor 652 to the secondary winding M6, the conductor llt also being connected through a part of resistor Hi8 and conductor Hill to the secondary winding l lli of transformer I44.

In order to control the direct-current biasing potential for the discharge valves 38-38 and fist-d2, vacuum type discharge valves 53d and E82, respectively, are provided being disposed to control the current flow through the potentiometer resistors H59 and i152, respectively. The valves i853 and i822 comprise anodes l't land B85, respectively, grids i135 and ltl, respectively, and cathodes Hill and i922, respectively. The anodes EM and liili are connected by conductors i236 and 5%, respectively, to an output terminal of associated rectiflers 2M and respectively, which are sup plied from transformers 2M and 2%, respectively, the primary windings of the transformers being connected across supply conductors 5d and 58.

The cathode We of discharge valve liiil is connected through a self biasing resistor 2%, a flexible lead connected to an intermediate tap of a resistor 2V2, resistor i812, conductor it, resistor iii, conductors l t and ill, potentiometer resistor Mill, conductor 2i and a smoothing re actor lit to the other output terminal of the rectifier 2%. In a similar manner, the cathode [92 of valve m2 is connected through a selfbiasing resistor Bill, a flexible lead 228 connected to an intermediate tap of a resistor 222, resistor 85, conductor S l, resistor we, conductors $8 and 96, potentiometer resistor 662, conductor 22 and a smoothing reactor 2'26 to the other output terminal of the rectifier 202.

In order to control the biasing potential of the grids S86 and i853 of the discharge valves lflfl and H32 respectively, the grids lull and E88 are connected so that their potential is controlled in response to the current flowing through the electrode l2 and the arc potential across the electrode arc. Thus the grid biasing circut of the valve its extends from the grid E86 through a grid resistor conductor 2%, series connected resistors i332 and conductor 2%, resistor iii, conductor iii, resistor i532, a part of resistor 2H2, flexible lead Ella and self-biasing resistor this to the cathode Hit.

The resistors 232 and are control resistors and are disposed to have a direct current voltage thereacross proportional to the are potential and the flow of current through electrode l2, re-- spectively. For this reason the resistor 2232 is connected across a rectifier bridge which is connected through transformer 22%, the terrninals of the primary winding of which are connected by conductors 28,2 and 24 5 to supply conductor 2t and the receptacle of the grounded furnace ill, respectively. The control resistor 23 l is also connected across a rectifier bridge 2% which is supplied by a transformer connected to be energized by the current transformer 25% in accordance with the cur ent flowing in conductor 26. The transformers 5. 38 are preferably of a one-to-one ratio and are employed primarily to prevent sneak circuits or effects on the grids of the discharge valves i861 and I82.

As illustrated the rectifier bridges 238 and 246 are connected in opposition whereby the direct current voltages across resistors 232 and 235 are of opposite polarity. Thus for any variation in the flow of current through the electrode and in the are potential, the diflerential oi the direct current voltages across the series connected resis-tors i232 and 23s normally controls the direct current biasing potential on the grid E86 of the discharge valve I30.

The grid I83 of the discharge valve I82 is likewise connected through a grid resistor 252, conduct-or series connected resistors 256 and 258, conductor 2%, resistor Hill, conductor ti l, resistor a part of resistor 222, flexible lead 228 and the sell-biasing resistor 2H8 to the cathode E92. The control resistor its is connected across a rectifier bridge the input terminals of which are con-- nected by conductors 2 42 and its to the supply conductor Bil and the receptacle of the grounded furnace ill, respectively, whereby a direct current voltage proportional to the arc potential is impressed across resistor 25$. The control resistor is also connected across a rectifier bridge 2%, the input terminals of which are connected across the current transformer 25% on the conductor ti l whereby the direct current voltage impressed across resistor 258 is proportional to the current flowing through the electrode l2. The rectifier bridges 252 and Mi l are so connected across the resistors 25B and 258, respectively, that the direct current voltages impressed thereacross are of opposite polarity for normally controlling the direct-current biasing potential of the grid H33 of the valve lSZ.

In the system described, the resistors 26S and Eli; are selected so as to assure a negative bias on the grids of the valves E and M2, respectively, when the electrode i2 is in an ideal or balanced operating position and the direct current voltages across the control resistors are so balanced that the differential of the direct current voltages are substantially zero. The resistors 50 and H32 in the circuits with the set of discharge valves 3-3i and 4042 respectively, are so adjusted by adjusting the flexible leads Hi4 and H2, respectively, that the drop across the section of the resistors Hill and IE2 in circuit with the grids of the valves at-ss and ill-M, respectively, gives a sufficient dead zone to prevent simultaneous firing of both sets of the discharge valves.

The resistors 75 and led in the motor armature circuits controlled by the firing of va ves 36-38 and lth-d2, respectively, are also connected in the grid circuits of the valves i823 and i132, respectively, and function to automatically limit the starting current of the motor to a safe value for the valves (ii -:38 and til-42. In addition to limiting the starting current of the motor, the resistors l6 and Hill also function to protect the va ves 3838 and it-52, respectively, from any overload on the system by so controlling the grid bias of the valves that the current flow to the motor is within the safe current limits of the valves. The specific manner in which such protection is obtained will be fully explained hereinafter in connection with the operation of the system.

In accordance with this invention, the resistors 86 and cooperate to permit normal operation of the motor 2t when the valves 36 and 38 are conductin and to provide automatically brak- 5. ing for themotor when the valves and 38 cease to pass current to the motor. The functioning of the resistors 86 and 222. will be explained more fully hereinafter.

The resistors I02 and H2 are associated with the valves 40 and 42 for functioning inthe same manner as resistors 86 and 222 to permit'normal operation of the motor 26 when the valves 40 and 42 are conducting and to provide automatic braking of the motor-when the current ceases-to fiow through the valves 4!] and 42 to the motor.

Inoperation, assuming that the system is energized, the electrodes I2, I4 and I5 are positioned with respect to the metal charge I8 in the furnace by a manual or automatic manipulation (not shown) of the electrodes, such positioning and methods of doing so being well known in the art. Assuming that the initial positioning of the electrodes is tobeobtained automatically, as soon as the system is energized. and conductors 2U, 22, and 24 are connected to a source of power supply, a potential exists from the electrode I2 to the. metal. IS in the furnace. This potential is at a maximum and since the electrodes I2. I4 and I6 are not as yet adjusted, there is no current flow. The high value of the arc potential impresses a high value of direct current voltage across each of the control resistors 232 and 255, and since current is not flowing through the electrode I2, the direct current voltages across. resistors 234 and 258, proportional to the flow of current are zero. The direct current voltage across control resistor 232 places a large negative bias on the grid I86 of valve I81! whereas the direct current voltage across control resistor 256 places a less negative or more positive bias on the grid I88 of valve I82.

The bias thus impressed on the grid I88 is such that the valve I82 is rendered more conductive with the result that current flows from the negative terminal of rectifier bridge 202, through reactor 226, conductor 224, potentiometer resistor I62, conductors 96. and 98-, resistor I03, conductor 84, resistors 86 and 22-2, flexible lead 220, self-biasing resistor 2J8, cathode I92 and anode I85 of valve I82 and conductor I98 to the positive terminal of the supply rectifier bridge 232. Since the direct current voltage proportional to the are potential is at a maximum,

the conductivity of the valve I82 is also a maxivalves 45 and 42, respectively, more negative whereby the grid-cathode potential of; the valves is lowered and maintained at a potential below the critical potential of the valves and the valves 45 and 42 are blocked with the result that no current flows therethrough to the motor 23.

At the same time, the large direct current voltage proportional to the arc potential impressed across control resistor 232 cooperates with the self-biasing resistor 258 to impress a more negative biasing potential on the grid I86. todecrease the conductivity of valve I80. Thus the current flow through potentiometer resistor I50 in the anode-cathode circuit of valve I 80 is so decreased that the direct current biasing potential for the valves 36 and 38 is so reduced that the resultant grid-cathode potential of valves 36 and 33 rises above the critical potential of the valves and they become highly conductive to pass current to the motor 25. This conducting circuit extends from the center tap at of the secondary winding of transformer 52 through conductor 88, resistor 86, conductors- 84--and 82',- ahmature windings 32: of the motor 26, conductors and I8, resistor I6, conductor I4, the parallelconductors Ill-453 and 12 to cathodes 6B and 62, respectively, of valves 35 and 38, anodes 44 and 45, respectively, and fr-om thenceto the terminals of the secondary winding of transformer 52, to so energize the motor 26 asto effect the operation thereof in a direction to lower the electrode I2, Since the other electrodes I4: and I5 are as yet not in contact with the metal bath I8, the electrode 12 is lowered to a position where it engages the metal I8.

Asthe-electrode I2 is lowered the potentialbetween theelectrode I2-and the metal I8 decreases with the result that the direct-current voltage across resistors 232 and 256 is so decreased that the bias on the grids of Valves I and I82 is rendered lessnegative and more negative, respectively. By rendering the biasing potential ofthe grid-=of valve I82 more negative the current flowing through the resistor I52 for controlling the biasin potential of the valves 45 and 42 more nearly approaches the value where the discharge devices 45 and 42 can be rendered conductive.

As soon as the electrodes I2, I4 and I6 are actuated to a position where a circuit is completed through the electrodes in conjunction with the metal I8, maximum current flows through the electrode I2 and a direct current voltage proportional to the current is imposed across each ofthe control resistors 234 and 258 with the result that the difierential of the direct current voltages across resistors 232-23$ and 256-458 is such as to place a less negative bias on the grid valve I89 and a more negative bias on the grid valve I82.

The lessnegative or more positive bias on the grid I85 of valve I85 renders the valve more conducting so that more current flows through the circuit extending from the negative terminal of the supply rectifier bridge 202} through the reactor 2H5, conductor 2I4 potentiometer resistor I55, conductors I6- and "I5, resistor conductor '55, resistors I 82 and-2 I 2, flexible lead self-biasing resistor 25%), cathode I and anode I84 of valve I89, and conductor I95 to the positive terminal a of the rectifier bridge 255. The current flowing to block the firing of the valves and 5G and prevent the flow of current therethrough to the motor 25-.

At the same time that valve I55 becomes conducting, the change in the biasing potential on the grid I83; of valve I82- is such as to decrease the current passed by the valve I82 with the result that the current flow through the potentiometer resistor I52 is so decreased or becomes zero, that the direct current biasing potential on the grids H2 and N4 ofvalves 45 and 42;, respectively, becomes less negative or more positive, and the valves 4!} and 42 become conducting to pass current to the motor 26. The motor circuit thus established extends from the center tap I55 of the secondary winding of-transformer through conductor I54, resistor I82, conductors If; and 8B, armature windings 3211fthe motor 2t, conductors 82 and, resistor I09, conductor 38', the parallel conductors 92 and 96 -9 4-, cathodes 64- and- 4-2, respectively,- and' anodes 48 and 55, respectively,

2,45a2oe of valves id and 32, respectively, to the terminals of the secondary winding of the transformer 1 to so energize the motor 28 as to efiect an operation thereof in a direction to raise the electrode l2.

The electrode i2 is thus raised to a position for ideal operation of the arc furnace, in which position the direct current voltage across each of control resistors 232 and 256 which is proportional to the are potential, and the direct current voltage across each of resistors 234 and 258 and which is proportional to the current flowing through the electrode l2, are balanced and a zero differential appears across the series connected resistors ESQ- .2% and the series connected resistors 256- Under such conditions the self-biasing resistors and 213% function to maintain sufficient bias on the grids of valves Ito and I82, respectively, whereby they are sufficiently conducting to pass sufficient current through potentiometer resistors liifi and H52, respectively, that the direct current biasing potential on the grids oi valves and ill-42, respectively, is sufficient to blocl; the valves from passing current to the motor f the ideal operating condition for the arc furnace id is changed in any manner whatsoever, the balance of the direct current voltages across control resistors and 256-258 is upset with the result that the bias on the grids of valves iiiil and W2 is changed to effect an operation of the motor in a predetermined manner to again establish the required balance between the our rent and potential across the arc. If, for example, the electrode iii should come in contact with the metal it as by reason of a cave-in of the metal, the arc potential is reduced to a minimum and the current flowing through the electrode is a maximum with the result that the differential or" the direct current voltages across control resistors 232 and 23 is of such a polarity as to render the grid of valve i536 more positive and thereby render the valve liill more conductive. As described hereinbefore, when the valve ISO is thus rendered more conducting the flow of current through potentiometer resistor Hi0 'so changes the grid-cathode potential of valves 36 and 38 as to render them non-conducting.

At the same time, the differential of the control voltages across resistors 258 and 252 is such as to place a more negative bias on the grid of valve 82 to decrease the current fiow through the valve 82. Under such conditions, the direct current biasing potential on the valves id and E2 is such that the valves are rendered less negative and current fiows therethrough to the motor 26 to cause its operation to raise the electrode i2.

During the operation of the system the resistors l8 and ice function to automatically limit the current flowing to the motor 23 each of the resistors being of low ohmic value and being connected in the motor armature circuits. As illustrated the resistor Hill is also connected in the anode-cathode circuit of valve 882 whereas the resistor i6 is also connected in the anode-cathode circuit of valve 589.

When the motor 26 is at rest the series connected resistors 222, 86 and Hill carry no motor current and therefore they have no effect on the control of valve E82. Likewise the resistors 212, M12 and 16 in the anode-cathode circuit of valve iiill carries no motor current when the motor 26 is at rest. If then, the direct current voltages across control resistors 23223 l and 256258 become unbalanced, as for example, whereby the differential of the direct current voltages is such as to cause a more negative voltage on the grid [$8 of valve I82, the negative bias from the associated potentiometer resistor i522 impressed on the grids of valves id and 322 is reduced and the motor current flowing therethrough will tend to reach a high value when the motor 2% is started from rest.

Under such conditions, the current flowin through the armature windings 32 also flows through resistor Hill which is also connected in the grid circuit of valve I82. As the flow of current to the motor 26 through resistor lid} increases, the potential across resistor ifii) is such as to apply a positive grid bias to the grid of valve I82 to ofiset some of the negative grid bias impressed as a result of the differential of the direct current voltages across control resistors 258 and 258 whereby the valve I82 becomes more conducting. As the valve I82 thus becomes more conducting, current flows through the potentiometer resistor IBZ to effect a change in the grid bias of valves 50 and 42 to phase back the firing angle of the valves 42 and 42 to effectively reduce the motor voltage and current. The resistor Hill is so selected that the potential drop thereacross is only effective at motor currents of a predetermined high amplitude or value, for example, such as are encountered at starting from rest or the like, to render the valve I82 more conducting. At all values of motor current below the selected and predetermined value, the resistor Hi0 is ineffective for rendering the valve I82 more conducting.

The resistor 76 functions in the same manner to limit the motor current if the differential of the direct current voltages across resistors 232 and 234 is such as to cause a more negative volt age on the grid I36 of valve l8i whereby the valves 36 and 38 are rendered conducting to pass current to the motor 25. Under such conditions the current flowing through armature windings 32 also flows through resistor 16 and where the motor current reaches a predetermined value, a positive grid bias from the resistor iii is impressed on the grid of valve H80 to offset some of the negative grid bias impressed thereon by the differential of the control voltages to render the valve Hill more conducting. The flow of current through the potentiometer resistor ifill thus resulting effects a change in the bias of valves 35 and 38 to phase back the firing angle of the valves 36 and 38 to eifectively reduce the motor voltage and current. The resistor 16 is selected in the same manner as resistor me so that the potential drop thereacross is only eliective at motor currents of a predetermined high value approaching the safe value for the valves 36 and 38 in so offsetting the negative bias of the differential to cause the valve I80 to conduct. At all values of motor current below the predetermined value, the positive biasing potential of resistor 76 is insufficient to overcome the negative biasing effect of the differential of the control voltage.

In all of the operations described hereinbefore, the resistors 86-222 and Il322 l 2 have a definite part as the valves 3$38 and 4El l2, respectively, pass current to the motor 26 and as the valves cease to conduct. For example, assume that the motor 26 is being supplied through the valves 36 and 38, to effect its operation to lower the electrode I 2. In such an event, a voltage drop appears across resistor 222 connected across conductors l4 and 88 which is proportional to the 9 voltage being impressed on the armature windings 32 of the motor 26. At the same time, a voltage drop of opposite polarity appears across resistor 86 which is proportional to the current supplied through valves 38 and 38 to the motor.

In practice, the flexible lead 220 is so adjusted that the voltage drop across the section of resistor 222 which connected between conductor 88 and the flexible lead 228 is substantially equal to the voltage drop across resistor 88 for the diiferent operating conditions of the motor 26. Therefore, as long as the valves 36 and 38 are conducting, the drop across resistor 86 ofisets or cancels the voltage drop across the section of resistor 2232 which is connected between the conductor 88 and flexible lead 220 and which is also connected in 'the grid cathode circuit of the discharge valve I82. Thus, the resistors 86 and 222 have no effect on the bias of the discharge valve I82 as long as the electric valves 3'5 and 38 are conducting.

As the motor 25 is operated to lower the electrode I2, it is to be noted that the differential of the control voltages across resistors 2322-234 and 256--258 approach zero with the result that the bias of discharge valves I83 and I 82 are accordingly changed to control the bias of the electric valves 3538 and 4042, respectively, As the valves Bil-38 cease to conduct current to the motor 26, the valves 40-42 are also normally biased below their critical voltage to also prevent the flow of current to the motor 25.

However, when the valves 38 and 33 cease to conduct current, the voltage drop across resistor 86 is removed and the potential across that section of resistor 222 in circuit with the grid of valve I82 functions to place a slightly more negative bias on the grid I88 of discharge valve I82. The more negative bias on the grid I88 so controls the discharge valve I82 that the current flowing in the potentiometer resistor IE2 is so decreased as to permit a slight positive bias on the grids of electric valves 4'0 and 42, whereby the valves 40 and 42 are ultimately rendered slightly conducting during the negative half-cycle and the power generated by the coasting motor 26 is supplied through the valves 48 and 42 to the supply conductors 55 and 58. By absorbing the kinetic energy of the coasting motor 26 in this manner, 'themotor 26 is efiectively braked and brought to rest.

If, on the-other hand, the motor 26 is being supplied through the valves 48 and 42, to effect an operation on the motor to raise the electrode 12, then a voltage drop appears across resistor 212 which is proportional to the terminal voltage of the motor 26 and a voltage drop of opposite polarity appears across resistor I 02 which is proportional to the flow of current to the motor 28. With the flexible leadZID adjusted so that the voltage drop across the section of resistor 2! 2 which is connected between the conductor I04 and the flexible lead "H is substantially equal and opposite to the potential across resistor I02, the resistors 2I2 and I02 are ineffective to modify "resistor 32 ceases to eXist, and the potential across the section of resistor 2I2 in the grid cathode circuit of the discharge valve I80, which potential is proportional to the terminal potential of the'coasting motor'28, is impressed on the grid I85 of discharge valve I80 to so control the bias thereof that the'curren't flow through'the potentiometer resistor IE8 is decreased, whereby the electric valves and 38 are rendered slightly conducting to connect the motor 23 to the source of supply 56 and 58. Under such conditions, the kinetic energy of the coasting motor is absorbed by the supply conductors and the motor is efiectively braked and brought to rest.

As will be appreciated, the negative bias appearing across the sec ion of resistors 222 and 2I2 in tLe rid cathode circuit of valves i82 and I80, respectively, is operative only during that period time after the valves 3838 and 4042, resp ctively, cease to conduct and the motor 26 is to a stop. At all other times, the grid bias discharge valves I and I82 is normally i. lied by the sources of control voltages associated. therewith, as explained hereinbefore.

As will be apparent with the system connected described, if the biasing potential of either of the sets 36-38 or id-42 cf electric valves becomes less negative, more current flows through the armature windings 32 of the motor 25, the increase in current effecting an increase in the speed of the motor. As the position of the electrode I2 is changed, the dih'erential of control voltages across resistors 232-4234 and 25ii258 is changed to control the bias on the grids of discharge valves i230 and I82, respectively, whereby the bias on valves M -88 and 4Eii2, respectively, is controlled to control the current passed to the motor 25 effectively slow the motor as the electrode 2 approaches the position Where a predetermined balance is obtained between the arc current and arc potential.

The system of this invention is effective to afford. automatic braking of the motor when one set or the other of the electric valves 3638 and 49-5-2 cease to conduct, such. braking being obtained without the use of mechanical relays or contactors. As the utilizes standard parts. and can readily be duplicated. it is apparent that the advantage of the raking system of this invention can readily be obtained without greatly adding to the investment of present systems.

I claim as my invention:

1. In a control system, in combination, a reversible motor the operation of which is to be controlled, a pair of valve means disposed to selectively connect the motor to a source of alternating current to control the direction of operation of the motor, the motor being normally disposed to coast when either of the valve means function to disconnect the motor from the source, a control means for rendering the pair of valve means selectively conductive, a source of control voltage responsive to the operation of the control means, and resistor means connected in circuit between each. of the valve means and the motor and interconnected with the control means to efiect an operation of the control means when one or the valve means ceases to conduct to render the other valve means conductive to connect the coasting motor to the source and thereby effect a braking of the motor.

2. In a control system, in combination, a reversible motor the operation of which is to be controlled, a pair of valve means disposed to selectively connect the motor to a source of alternating current to control the direction of operation of the motor, the motor being normally disposed to coast when either of the valve means function to disconnect the motor from the source, a control means for rendering the pair of valve means selectively conductive, a source of control voltage responsive to the operation of the motor for nor- :nally controlling the operation of the control means, and resistor means connected in circuit between each of the valve means and the motor and interconnected with the control means to effect an operation of the control means when one of the valve means ceases to conduct to render the. other valve means conductive to connect the coasting motor to the source and thereby effect a braking of the motor, said interconnected resistor means being effective to render the other valve means conductive only during the period of time existing after the one of the valve means ceases to conduct and until the motor is braked to a stop.

3. In a control system, combination, a reversible motor the operation of which is to be controlled, a pair of valve means disposed to selectively connect the motor to a source of alternating current to control the direction of operation of the motor, the motor being normally disposed to coast when either of the valve means function to disconnect the motor from the source, a control means for rendering the pair of valve means selec tively conductive, a source of control voltage responsive to the operation of the motor for normally controlling the operation of the control means, and means comprising a pair of resistors connected in circuit relation between each of the valve means and the motor and interconnected with the control means, one of the resistors being disposed to comprise a source of potential propor tional to the current flow to the motor when the valve means connected in circuit therewith conducting, the other resistor being disposed to comprise a source of potential proportional to the terminal potential of the motor, the sources of proportional potential being substantially equal and opposite when the valve means connected in circuit therewith is conducting, the source of potential proportional to the terminal potential of the motor being effective when the valve means con.- nected in circuit therewith ceases to conduct to render the other valve means conducting to corn nect the coasting motor to the source of alter hating current and thereby efiect a braking of the motor.

4. In a control system, in combination, a reversible motor the operation of which is to be controlled, a pair of valve means disposed to selectively connect the motor to a source of alternating current to control the direction of opera-- tion of the motor, the motor being normally disposed to coast when either of the valve means function to disconnect the motor from the source, a control. means for rendering the pair of valve means selectively conductive, a source of control voltage responsive to the operation of the motor for normally controlling the operation of the corn trol means, and means comprising a pair of resistors connected in circuit relation between each of the valve means and the motor and interconnected with the control means, one of the resistors being disposed to comprise a source of potential proportional to the current flow to the motor when the valve means connected in circuit therewith are conducting, the other resistor being disposed to comprise a source of potential proportional to the terminal potential of the motor, the sources of proportional potential being substantially equal and opposite when the valve means connected in circuit therewith is conducting, the source of potential proportional to the terminal potential of the motor being effective when the valve means connected in circuit therewith ceases to conduct to render the other valve means conducting to connect the coasting motor to the source of alternating current and thereby effect a braking of the motor, the source of potential proportional to the terminal potential of the motor being effective to render the other valve means conductive only during the period of time existing after the valve means connected in circult with the source of proportional potential ceases to conduct and until the motor is braked to a stop.

5. In a control system, in combination, a re versible motor the operation of which is to be controlled, a plurality of pairs of electric valves disposed to selectively connect the motor to a source of alternating current to control "he direction of operation of the motor, the electric valves having grids to be utilized for controlling the conductivity thereof, a grid control. circuit including a resistor for each pair of the electric valves, a discharge valve having a control grid therein to be utilized for controlling the bias of the grids of the pair of electric valves associated therewith, a pair of sources of control voltages variable in opposite senses in response to the operation of the motor for impressing predetermined biases on the grids of the discharge valves, a resistor connected in circuit with each pair or" electric val es disposed to comprise a source of potential proportional to the current flow to the motor when the pair of electric valves associated therewith are conducting, and a resistor connected in circuit relation with each pair of electric valves disposed to ccmprise a source of potential proportional to the terminal potential of the motor, the potential. of the sources associated with each pair of electric valves being substantially equal but of opposite polarity when the associated pair of electric valves is conducting, the resistors associated with each pair of electric valves and comprising the sources of potential also being connected in series circuit relation in the grid circuit of the discharge valve not associated with said pair of electric valves to render the unassociated discharge valve non-conducting and the pair of electric valves controlled thereby conducting when the other pair of electrio valves are non-conducting to efiectively brake the motor.

6. In a control system, in combination, a reversible motor the operation of which is to be controlled; a first and second pair of electric valves disposed to selectively connect the motor to a source of alternating current to control the direction of operation of the motor; the electric valves having grids to be utilized for controlling the conductivity thereof; a grid control circuit including a resistor for each pair of the electric valves; a first and second discharge valve having a grid in each disposed to be utilized for controlling the bias of the grids of the first and second pairs, respectively, of electric valves; a first and a second pair of sources of control voltages variable in opposite senses in response to the operation of the motor for impressing predetermined biases on the grids of the first and second discharge valves, respectively; a first and second resistor connected in circuit with the first and second pairs, respectively, of electric valves disposed to have potentials thereacross proportional to the fiow of current therethrough to the motor; and another first and. second resistor connected in the grid circuit of the second and first discharge valves, respectively; said another first and second resistors also being connected across the input circuits to the motor from the first and second pairs, respectively,

13 of electric valves to a potential thereacross proportional to the potential across the motor; the potentials across the first and second resistors in circuit with the first and second pairs. respectively, of electric valves being substantially equal but opposite to the potentials across said another first and second resistors, respectively, connected in the grid circuits of the second and first discharge valves, respectively, when the first and second pairs, respectively, of the electric valves are conducting; said another first and second resistor cooperating in the grid circuits of the second and first discharge valves, respectively, to render the respective discharge valves non-conducting when the first and second pairs, respectively, of electric valves cease to conduct and the motor is coasting whereby the second and first pairs, re-

spectively, of electric valves are rendered conducting to pass current from the coasting motor to the source of alternating current and thereby effect the braking of the motor.

JACK E. REILLY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,449,896 Evans Mar. 27, 1923 2,248,020 Freudenhammer July 1, 1941 2,399,363 Levy Apr. 30, 1946 

